PCI: rewrite PCI BAR reading code

Factor out the code to read one BAR from the loop in pci_read_bases into
a new function, __pci_read_base.  The new code is slightly more
readable, better commented and removes the ifdef.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>